This invention relates to an integrated circuit having a fuse circuit, and in particular, to a fuse circuit which detects whether the fuse has been blown or not by judging whether the fuse resistance is larger or smaller compared to that of a predetermined threshold resistor.
An integrated circuit (IC) having fuse circuits is becoming increasing popular. It is well known that the characteristics of an IC may be above or below its rated characteristics due to fluctuation of its fabrication process. For example, the characteristics of an IC having a built-in reference voltage may vary from its rated characteristics because of such fluctuations. In order to reduce such variations, each IC chip has been trimmed in the prior art. The chip trimming is often performed by a fuse circuit. For example, the fuse circuit is connected to a D/A converter circuit, to control precisely its reference voltage by the fuse circuit. The fuse circuit is capable of adjusting the gain between the input and output signal by blowing the fuse by applying a high current.
FIG. 1 is a conventional fuse circuit for trimming. In the figure, a first field effect transistor (FET) Tr1 is an n-channel depletion type transistor. In order to provide a function of constant current source, the gate electrode G1 of the first FET Tr1 is connected to its source S1. A fuse circuit is formed by the first FET Tr1 and a fuse F, which are connected in series between a positive voltage source V.sub.DD and a negative voltage source V.sub.SS. A junction point of the source S1 of the first FET Tr.sub.1 and the fuse F is connected to an input terminal of an inverter INV for example which detects whether the fuse is blown or not. The inverter INV is composed of a complementary metal oxide semiconductor (CMOS) transistors including an enhancement type p-MOS transistor Tr2 and an n-channel MOS transistor Tr3.
The fuse F is blown by applying a high current at terminal pads PAD. A and B. The judging of whether the fuse F has been blown or not is done by checking whether an on-state resistance of the first FET Tr1 is larger or smaller compared with a resistance R.sub.F of the fuse F. Namely, when the fuse F is blown a judging potential Vp at the junction point rises up to a high potential level "H", and it is applied to the input of the inverter INV, for example. The output of the inverter INV then becomes low potential level "L". On the contrary, if the fuse F has not been blown, the judging potential Vp at the junction point is the same potential of the negative voltage source Vss, namely the low potential level "L". Then an output of the inverter INV becomes high potential level "H".
Generally, in such a conventional fuse circuit, in order to decrease a power dissipation of the fuse circuit, the on-state resistance of the first FET Tr1 is designed to be larger than that of the fuse F. For example, the fuse F has a small resistance of about 100 ohms when it is in the unblown state, but an internal resistance between the drain D1 and the source S1 of the first FET Tr1 is about a few Meg-ohms. Usually, the fuse circuits of the prior art are used in such a condition. After the fuse has been blown, occasionally, a gap of the fuse terminals becomes leaky in performance. This is so called as "re-glown fuse" or "glown-back fuse". A change of the resistance R.sub.F is caused by moisture or contamination of the gap of the fuse terminals. For example, when the resistance R.sub.F of the fuse F is decreased from 3 meg-ohms to 2 meg-ohms by the glown back fuse, the input of the inverter INV is changed from the high potential level "H" to low potential level "L". Thus, the fuse circuit operates erroneously, and the trimming circuit is out of control.
Therefore, in order to bring the conventional fuse circuit to a desired level, it is required to provide the IC with a fuse which does not malfunction by leaking, and it is desirable to provide a circuit which detects the fuse condition.